Polypropylene resin is used in various fields, such as fields of miscellaneous goods for daily use, kitchen utensils, packaging films, automotive parts, mechanical parts and electrical parts, as a molding material for them, and this resin is practically used in the form of a composition further containing various compounding agents and additives which are added according to the properties required for each commercial product. For example, in fields where mechanical strength is required, such as a field of automotive exterior trim, polypropylene resin compositions in which elastomer, talc and the like are added are employed.
On the other hand, it is regarded as important that the automotive exterior trim has a united impression with automotive body in design, and to meet this, the same color coating as that for the body has been carried out on the molded articles. Polypropylene, however, is poor in the coating properties, so that primer coating and drying are necessary. Also from the viewpoints of VOC reduction and energy saving in recent years, polypropylene resin compositions capable of omitting the primer coating process have been desired.
As a method for imparting favorable coating adhesion properties to a polypropylene resin without performing primer coating of the polypropylene resin, Japanese Patent Publication No. 64660/1993 (Japanese Patent Laid-Open Publication No. 272255/1986) describes a method of mixing polypropylene having been modified with an organic compound having a hydroxyl group. However, if favorable coating adhesion properties are intended to be maintained, the strength inherent in the polypropylene resin tends to lower.
To solve such a problem, the following means have been proposed. For example, Japanese Patent Laid-Open Publication No. 41276/1996 describes, as a coating propylene resin composition capable of maintaining a balance of high mechanical strength (rigidity, impact resistance) and excellent molding processability, a composition comprising a propylene/ethylene block copolymer, a styrene elastomer or an ethylene/butene-1 bipolymer rubber, a modified propylene polymer having been grafted with maleic anhydride or a hydroxyl group-containing maleic anhydride derivative having a specific structure, and a filler, wherein the modified propylene polymer contained is a modified propylene polymer obtained by thermally oxidizing a propylene/ethylene random copolymer having a number-average molecular weight of 7,000 and an ethylene content of 3% by weight, grafting maleic anhydride in an amount of 10% by weight based on the whole amount by the use of an azo type radical initiator and then subjecting the resulting polymer to secondary modification with ethanolamine. Especially in the case where a balance between high rigidity and impact resistance and coating adhesion properties are obtained, however, there is substantially room for improvement in the molding processability.
Japanese Patent Laid-Open Publication No. 101891/1998 describes, as a resin composition which has high impact resistance and high coating properties and is not lowered in the impact strength after coating, a resin composition comprising a specific propylene/ethylene block copolymer, an ethylene/octene rubber, a triblock copolymer, a modified polyolefin polymer having a hydroxyl group, and talc, wherein the modified polyolefin polymer having a hydroxyl group contains 0.5 to 7% by weight of an unsaturated compound having a hydroxyl group and the modified polyolefin polymer is added in an amount of 0.5 to 20 parts by weight based on the whole amount of the composition. In this publication, moreover, a method for obtaining high impact resistance by the addition of a relatively small amount of modified polyolefin is proposed. However, especially when high impact resistance and coating adhesion properties are obtained, primer coating is substantially necessary.
From the viewpoint of VOC reduction in recent years, improvement of coating adhesion efficiency due to electrostatic coating has been made, and in the existing circumstances, use of a conductive primer generally makes it possible to perform electrostatic coating. In the use of a conductive primer, however, there is a problem of poor color development of the intermediate coating material, and also from the viewpoint of decrease of cost required for the coating process, it has been desired to omit the conductive primer coating process. That is to say, a polypropylene resin composition which can impart conductivity to a material used for a molded article to be coated and which exhibits favorable coating adhesion properties even if primer coating is not carried out is desired.
As a method to impart conductivity to a thermoplastic resin, filling of carbon black has been heretofore known, and with increase of a fill of carbon block, the conductivity becomes better, but the strength and the fluidity inherent in the thermoplastic resin tend to lower. To solve such a problem, the following means have been proposed. For example, Japanese Patent Laid-Open Publication No. 279310/1996 describes, as a conductive resin composition free of variability of the appearance of a molded product, mechanical strength and conductivity by the use of specific polypropylene, polyethylene and carbon black, an injection molded product comprising a master batch and a polypropylene resin in an amount of 60 to 200 parts by weight based on 100 parts by weight of the master batch, said master batch containing a polypropylene resin having a melt flow rate of 10 to 50 g/10 min in an amount of 50 to 90 parts by weight, a polyethylene resin having a density of not more than 0.910 g/cm3 in an amount of 10 to 50 parts by weight and carbon black having a DBP oil absorption of 60 to 200 ml/100 g and a specific surface area of 30 to 200 m2/g in an amount of 60 to 150 parts by weight based on 100 parts by weight of the total of both the resins.
Japanese Patent Laid-Open Publication No. 231824/2003 describes, as a conductive thermoplastic resin imparted with higher conductivity by the addition of a smaller amount of specific carbon black and having smaller variability of conductivity as compared with conventional carbon black, a high-conductivity thermoplastic resin composition containing carbon black having a micro pore width of 4.0 to 8.0 angstroms and a micro pore volume maximum value of 0.06 to 0.135 ml/angstrom/g.
Japanese Patent Laid-Open Publication No. 65064/1985 describes, as a high-conductivity carbon black-containing composition imparted with high conductivity and stability by the addition of specific carbon black, a high-conductivity carbon black-containing composition comprising 30 to 97% by weight of a synthetic resin or a synthetic rubber and 70 to 3% by weight of high-conductivity carbon black which has a DBP oil absorption of not less than 350 ml/100 g and is prepared simultaneously with formation of a synthetic gas by a process comprising subjecting liquid hydrocarbon to partial oxidation reaction with molecular oxygen and water vapor in a furnace, in said process a weight ratio of carbon atom/hydrogen atom in the hydrocarbon being not less than 9 and the furnace being operated under the conditions of a furnace internal temperature range of 1300 to 1450° C., a furnace internal pressure of 25 to 80 kg/cm2 and an amount of water vapor fed to the furnace of 400 to 800 kg based on 1 ton of the hydrocarbon.
In the fields where high qualities are desired, such as a field of automotive exterior trim, however, further improvement in a balance among processability, mechanical strength and conductivity is necessary.